A promising strategy to induce an immune response capable of neutralizing papillomavirus (PV) infections is the use of virus capsid proteins as antigens. In the case of genital human papillomaviruses (HPVs), this approach was hampered by the lack of any in vivo or in vitro source of sufficient amounts of native virus. In order to overcome this problem, heterologous expression systems have been extensively used to obtain large quantities of capsid proteins and to allow the analysis of their structural and immunological properties. Expression of the major capsid protein late 1 (L1) from different PV types using prokaryotic (25), baculovirus (21, 23, 37, 41, 42, 46), yeast (14, 18, 19, 20, 29) and mammalian expression systems (15, 16, 51), demonstrated that this protein can self-assemble into virus-like particles (VLPs). Coexpression of the minor capsid protein late 2 (L2) is not strictly necessary to obtain VLPs, although its presence increases the efficiency of particle formation (15, 22, 51) and induces anti-L2 neutralizing antibodies (32). The L1 and L2 VLPs appear similar to native virions by electron microscopy (EM). The use of different animal models has shown that VLPs can be very efficient at inducing a protective immune response.
VLPs meet many of the criteria which make them ideal surrogates of native virions. They resemble infectious particles by ultrastructural analysis (16), elicit virus neutralizing antibodies and bind to the putative receptor on the surface of mammalian cells (28, 31, 33, 44, 47). Most notably, the results obtained with animal models demonstrated that prophylactic immunization with VLPs can be very effective in vivo. Cottontail rabbits, calves and dogs immunized with L1 VLPs were protected from subsequent challenge with the homologous PV (20, 23, 41) and passive transfer of immune sera conferred protection to naive animals (20, 41), indicating that an antibody-mediated response plays a major role in preventing virus infection.
Studies with infectious HPV virions, as well as VLPs of different HPV types, strongly suggested, however, that the immune response is predominantly type-specific. Further, the efficacy of VLP-based anti-HPV vaccine candidates cannot be evaluated in animals since these viruses exhibit a high degree of species specificity. Antibody-mediated virus neutralization has been therefore studied using either in vitro assays (35, 40) or xenograft systems which allow propagation of infectious virus of specific HPV types (1, 2, 5, 6, 24). The primary conclusion which could be drawn from these experiments was that immunization with HPV VLPs evokes a neutralizing immune response which is predominantly type-specific (6, 7, 34, 35, 36, 48).
Cross-neutralization has been reported between HPV-6 and HPV-11 (92% amino acid sequence identity) (8) and between HPV-16 and HPV-33 (80% amino acid sequence identity) (48). This may indicate the existence of some correlation between protein sequences and structural similarities that could possibly be relevant for the mechanism of capsid assembly. On the basis of these considerations, however, the concept that HPV-6 and HPV-16 L1 proteins may coassemble is not obvious, since the two viruses belong to phylogenetically more distant groups (3, 45) and exhibit a lower (67%) L1 amino acid sequence identity.
Further, while envelope proteins of viruses belonging to very different families can be incorporated into the same envelope (50), nucleocapsid protein mixing appears to be much more restricted. Mixed core particles between Moloney murine leukaemia virus (MuLV) and human immunodeficiency virus (HIV) have been obtained but only when artificial chimeric Gag precursors, containing both HIV and MuLV determinants are coexpressed with wild-type MuLV Gag proteins (10). By using a yeast two-hybrid system based on GAL4-Gag fusion protein expression plasmids, Franke et al. were able to show that the ability of two heterologous Gag proteins to multimerize was correlated with the genetic relatedness between them (13).
Mixed capsid formation between wild-type Gag proteins has not been reported so far. In the case of the hepadnavirus core (C) protein, Chang et al. (4) have shown that an epitope-tagged truncated hepatitis B virus (HBV) C polypeptide could coassemble in Xenopus oocytes with woodchuck hepatitis virus (WHV) and ground squirrel hepatitis virus (GSHV) C proteins but not with that of duck hepatitis B virus (DHBV). This result was not unexpected since the two core protein sequences have diverged significantly and do not show immunological cross-reactivity. When coassembly of C polypeptides of HBV, WHV and GSHV occurred, formation of mixed capsids resulted from the aggregation of different species of homodimers (4).
Several reports have discussed the importance of disulfide bonds for the integrity of native bovine papillomavirus type 1 (HPV-1) virions (26) and VLP structures (25, 38, 39). Li et al. (26) have also shown that the cysteine 424 mutant (C424) of HPV-11 L1 in the carboxy-terminal domain that has been identified as critical for capsid formation (25), is still able to form capsomeres but not VLPs, indicating that this residue may be involved in interpentamer bonding. The essential role of disulfide bonds has been confirmed by a single point mutation of either C176 or C427 in HPV-33 L1 (C428 in HPV-18 L1), which converts all VLP trimers into monomers, allowing capsomere formation but not VLP assembly (39).
It has been recently proved that, by using an in vitro infection system and a sensitive reverse transcriptase PCR-based assay (RT-PCR), antisera to HPV-6 VLPs are not able to neutralize authentic HPV-16 virions (48). Since cysteine residues corresponding to those described as involved in disulfide bonding above are conserved in the HPV-6 and HPV-16 L1 proteins, we hypothesized that mosaic VLPs could either result from intra-capsomeric or inter-capsomeric association of the two proteins and/or from interaction between type-specific subsets of capsomeres.